Light-emitting diodes (LEDs) formed using semiconductor growth on monolithic wafers can demonstrate significantly higher levels of efficiency compared to incandescent sources. In this specification LED refers to an unpackaged LED die (chip) extracted from a monolithic wafer, i.e. a semiconductor element. This is different from packaged LEDs which have been assembled into a package to facilitate subsequent assembly and may further incorporate optical elements such as a hemisphere which increases its size and light extraction efficiency.
In lighting applications, the light from the emitter is directed using a luminaire optical structure to provide the output directional profile. The angular intensity variation is termed the directional distribution which in turn produces a light radiation pattern on surfaces in the illuminated environment. Lambertian emitters flood an illuminated environment with light. Non-Lambertian, directional light sources use a relatively small source size lamp such as a tungsten halogen type in a reflector and/or reflective tube luminaire, in order to provide a more directed source. Such lamps efficiently use the light by directing it to areas of importance. These lamps also produce higher levels of visual sparkle, in which the small source provides specular reflection artefacts, giving a more attractive illumination environment. Further, such lights have low glare, in which the off-axis intensity is substantially lower than the on-axis intensity so that the lamp does not appear uncomfortably bright when viewed from most positions.
Directional LED elements can use reflective optics (including total internal reflective optics) or more typically catadioptric type reflectors, as described for example in U.S. Pat. No. 6,547,423. Catadioptric elements employ both refraction and reflection, which may be total internal reflection (TIR) or reflection from metallised surfaces. A known catadioptric optic system is capable of producing a 6 degree cone half angle (to 50% peak intensity) from a macroscopic LED comprising a 1×1 mm light emitting element, with an optical element with 20 mm final output diameter. The increase in source size arises from conservation of brightness (étendue) reasons. Further, such an optical element will have a thickness of approximately 10 mm, providing a bulky illumination apparatus. Increasing the cone angle will reduce the final device area and thickness, but also produces a less directional source.
The LED of this example may be replaced by a 10×10 array of LEDs each for example 0.1×0.1 mm size, providing the same emitting area. This arrangement has a number of performance advantages, including reduced junction temperature (reducing illumination apparatus cost), reduced optical element thickness (reducing illumination apparatus cost), reduced current crowding (increasing device efficiency or reducing cost for a given output luminance) and higher current density capability (increasing device luminance or reducing cost for a given output luminance). It is therefore desirable to reduce the LED size.
It is desirable to reduce the number of electrical connection steps in connection of such an array of LEDs, to reduce cost. It is further desirable to reduce the area of electrical connection to such LEDs, preferably at least in proportion to the reduction of area of the LED to maximise emitting area of the chip. It is further desirable to provide electrical connections to LEDs on opposite surfaces to reduce current crowding.
PCT/GB2009/002340 describes a method to form an illumination apparatus with an array of small LEDs by preserving the separation of the LED elements from the monolithic wafer in a sparse array and aligning to an array of optical elements. GB2463954 shows one electrical connection method to LEDs of an LED array, in which the optical input aperture is positioned between the electrical connections and output aperture of the optical elements of the array of optical elements.
EP1 890 343 describes LEDs positioned in reflective cups with an overcoating transparent layver. Such devices are not suitable for providing directional illumination with narrow cone angles.
In addition, other objects, desirable features and characteristics will become apparent from the subsequent summary and detailed description, and the appended claims, taken in conjunction with the accompanying drawings and this background.